The present invention relates to dual layer membranes for selectively admitting gas components to gas component sensors.
Prior art electrolytic sensors for carbon dioxide have disclosed the following useful relationship. As disclosed in “Study of a new solid electrolyte thin film based micropotentiometric carbon dioxide gas sensor” (A. Essalik et al, J. New Mat. Electrochem. Systems 1, p. 67–70 (1998)) electrode reactions giving the EMF of such a sensor are as follows:
sensing electrode:1/2 02+2e−+2 Na+Na20CO2+Na20Na2CO3
reference electrode:AgAg++1e−
where Na+ and Ag+ are the mobile ions and Na20 and Na2CO3 are in solid state. The cell EMF can be written according to the Nernst equation as:EMF=K−[(2.3RTLogaAg+)/F]+[(2.3RTLogPO2)/4F]+[(2.3RTLogPCO2)/2F]
where K is a constant, F and R are the Faraday and gas constants respectively and T is the temperature. According to this equation, at constant PO2 and silver-ion activity aAg+, the EMF depends only on the CO2 partial pressure.
Also disclosed therein is an inherent restriction on the usefulness of that prior art electrode. “However, for practical use, stability of the sensors should be improved.” (Essalik et al, p. 70) and the article explained that the sensor lasted only a few hours at operating temperature. This limitation is a common problem of prior art electrolyte based carbon dioxide sensors. Typically, in other prior art carbon dioxide sensors, high temperature operation (400–500° C.) has been required, although the Essalik et al sensor displayed superior operational response at about 250° C.
There is a need for a carbon dioxide sensor after the Essalik et al device for which stable operation is maintained over a long period of time, sufficiently long for application to control or sensing systems wherein low power, low temperature carbon dioxide sensing may used to advantage.